Does "Show Boat" belong in the opera house? Well, of course it does - as long as it's done with the kind of theatrical and musical flair that the San Francisco Opera lavishes on its current production, which opened Sunday afternoon in a crowd-pleasing flurry at the War Memorial Opera House.

David Gockley, the company's general director, has been a little defensive on the subject of genre. And that's understandable, given the ease with which some listeners turn up their noses at this classic of the American musical stage, with its timeless songs by Jerome Kern and Oscar Hammerstein II and its panoramic view of the nation's social history.

But on Sunday it only took a few minutes of the overture, gleefully led by conductor John DeMain, for the audience to know that we were in for something both wonderful and fitting. It wasn't just that "Show Boat" can obviously take its place alongside such classics of operetta as "Die Fledermaus" or "The Merry Widow" - though that is true - but that the whole debate seemed beside the point in the face of the work's riches.

A work of layers

Those riches include not only the score's perennial musical jewels - chief among them "Ol' Man River" and "Can't Help Lovin' Dat Man" - but the ambition and sweep with which the show attempts to encompass a whole slice of American life through the prism of the theater. For all the easy charm and occasional pathos of Kern's music, this is a piece with more than just entertainment on its mind.

Yet entertainment - in the shape of the Cotton Blossom, the traveling Mississippi theater-on-the-river that gives the work its title - is the central metaphor here, and one of the virtues of director Francesca Zambello's splashy, energetic production is that it keeps at least one eye firmly fixed on the audience's pleasure.

This "Show Boat" offers a broad array of theatrical delights. There are rollicking dance numbers (the vivacious choreography is by Michele Lynch) and big crowd scenes, balanced by moments of intimate emotion and sheer romance. There is physical comedy and verbal comedy, bright bursts of color, hokey melodrama and an adorable fourth-grader whose singing can melt your heart within moments.

And underlying that variety is a strain of urgent, unflinching commentary on the racial injustices of the post-Civil War South. To hear the weary, noble strains of "Ol' Man River" in their dramatic context - especially in a shatteringly great performance by bass Morris Robinson - is to understand how far "Show Boat" can sometimes go beyond pure entertainment, and to get some inkling of the effect it must have had on New York audiences in 1927.

Of course, there are aspects of "Show Boat" that not even Zambello's brisk theatrical imagination can quite rescue - most notably, the extent to which the piece falls apart dramatically after intermission.

Throughout the first act, the show's lineaments, though intricate, are clear and effective, as we follow characters from three social strata. Magnolia Hawks, the daughter of the Cotton Blossom's proprietor and the eventual star of its watery musicales (soprano Heidi Stober, singing with vivid, bright-toned charm), is an ingenue straight from the world of operetta, and her romance with the dashing but suspect riverboat gambler Gaylord Ravenal (baritone Michael Todd Simpson, in a formidable and vocally robust company debut) plays out in fragrant love duets.

At the bottom of the social scale, where they have plenty of company, are the black characters who labor along the Mississippi - not only the stevedore Joe, whose "Ol' Man River" gives the piece its moral center - but the Cotton Blossom's cook, Queenie (in a radiant and witty performance by Angela Renée Simpson). And in between is Julie La Verne (soprano Patricia Racette, singing with fervor and clarity), whose career as the Cotton Blossom's prima donna comes to a hasty end when the secret of her racially mixed heritage comes to light.

The dramatic interplay among these characters is concentrated - along with plenty of comic diversion - in the first act, leading up to Magnolia and Gaylord's marriage. But then the action shifts to Chicago in the early 20th century, a milieu about which the show's creators have nothing interesting to say, and scrambles across two decades in an ungainly sprint toward an emotionally pointless conclusion.

An emotional score

Still, if the final curtain is apt to catch audiences up short, it will also leave them with plenty of happy memories. The emotional and formal range of Kern's score - from the catchy, poignant "Why Do I Love You?" to the exuberantly bawdy comedy of "Hey, Fellah!" (Queenie's lesson on how to catch a man) - provides a ramble through the pages of the American songbook, and DeMain shaped the performance with elegance and flair.

Bay Area favorite Bill Irwin brought his trademark loosey-goosey physical shtick to the role of Cap'n Andy, the Cotton Blossom's owner, and was ably joined by Harriet Harris as his domineering wife, Parthy Ann. Kirsten Wyatt and John Bolton provided a burst of music-hall pizzazz as Ellie Mae Chipley and Frank Schultz, the Cotton Blossom's second-banana team (Wyatt's version of "Life Upon the Wicked Stage," upending dreams of the glamorous theatrical life, was a keeper). Young Carmen Steele shone in her brief moment as Magnolia and Gaylord's 8-year-old daughter Kim.

But in the end, the show's indubitable star, and the standout among a host of fine performers, was Robinson, who has been heard only once before in San Francisco (in "Don Giovanni" two years ago). Singing with an extraordinary blend of power and fluidity, and projecting with effortless ease throughout the reaches of the house, he made Joe into an unshakable icon of humanity and pride, leavened with a delicate touch of humor. It was an unforgettable performance.